Interference suppression system for radio receivers and the like



Aug. 30, 1949.

A. J. H. OXFORD 2,480,599 INTERFERENCE SUPPRESSION SYSTEM FOR RADIO RECEIVERS AND THE LIKE Filed July 31, 1947 2 3 4 I I I l RE Amp f V LF. Amp? V Output V TQL Mixqr End 00!. Amp:

Mutiny Pulse I I Gen A. J. H. xford [nven or A fforney INTERFERENCE SUPPRESSION SYSTEM RADIO RECEIVERS AND THELIKE Alan John Henry Oxford, Christchurch, England, assignor to the Minister of Supply, in His Majestys Government of the-United Kingdom of Great Britain and Northern Ireland, London, England Application July 31, 1947, Serial No. 765,122

In Great Britain November 30, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires November 30, 1965 11 Claims. (01. 250-20) This invention relates to the reduction or elimmust introduce 8 additional muting pulses in orination of interference in radio telephone receivder that the interference should be inaudible. ers and other telephone channels due to pulse Ifthese muting pulses are 11 microseconds wide transmission. we have a tolerance of plus or minus 5 microsec- Difiiculty has often been experienced in oper- 5 ends to allow for any inaccuracies of timing. ating communication receivers in close proximity The time-interval of 9000 cycles is approximately to 'pulse radar transmitters owing to break- 110 microseconds, so that we are removing through of the pulse into the communication per cent of the signal time but this is insufficient channel, to prevent the normal operation of the radio re- The interference may be in the form of direct 10 ceiver. V break-through at the radar frequency of its har- The invention is illustrated by the accompanymonics. More frequently it is due to radiation of ing drawings given by way of example in which: the higher harmonics of the modulator pulse. Fig. l is a block schematic diagram of a com- Screening is found to be ineffective in many cases munication receiver embodying the invention and where the communication receiver is required to 15 Fig. 2 is a more detailed circuit diagram of a operate close to the radar transmitter. part of the receiver shown in Fig. 1.

Whatever form the interference takes, its ef- The receivershown in Fig. 1 comprises R. F. fect is to introduce a series of pulses at the recurand mixer stages I, I. F. and second detector rence frequency of the radar system which at stages 2,-an output stage 3 and telephones or loudthe outset are say, one or two microseconds long, speaker 4. A muting pulse generator 5 is conbut which due to the limited bandwidth of the trolled by lock pulses (corresponding to the inreceiver, and due to the effects of paralysis etc., terfering pulses) applied over a connection 6 and produce a note at recurrence frequency having produces a train of pulses of about 11 microa considerable mark-space ratio and therefore seconds pulse duration and 9000 C. P. S. recuran appreciable power tent. rence frequency which are applied in negative If the receiver be muted for a period embracoing sense to the control grids or suppressors of ing the occurrence of the pulse, 1. e. from a few the R. F. amplifier valves and the output valve microseconds before to a few microseconds after, or Valves in stages I and 3. The effect of the this pulse can be eliminated but a similar recurmuting p se is to bias back the controlled valves rence frequency note is still introduced due to the of the receiver and so mute the receiver for the muting pulse. duration of the pulse. The lock pulse applied It is now proposed that, in addition to the mutover connection 6 may be taken directly from ing pulse referred to above, a number of extra the modulator of the radar transmitter or may muting pulses should also be supplied equally be received on an elementary auxiliary receivspaced in time and making in conjunction with e1 1. r r

the original muting pulse, a continuous train hav- Fig. 2 of the accompanying drawing is a ciring a frequency higher than the audible limit cuit diagram of one form of muting pul required in the r iver, erator 5 for the system of Fig. 1. I

In general it will be found that this entails Referring to a p ode Valve Vl and silencing th receiver for a sma11 proportion of 40 associated resistances and capacities including the total time and therefore, it will still funccondenser l between anode and nd nd tion normally. a condenser C2 between second and third grids As it is desirable to avoid any broadening of constitutes a transitron circuit generating at'its the incoming interference pulse the muting is a d a p v going s -t vo o pdone in an early stage of the receiver. As, howl y 0 C e D Se d d at ts ever, the radiation is such that it may also be second 01' creen grid'a train of posit ve goin picked up in the low-frequency amplifier, this pulses coinciding with the rapid returns of the amplifier is preferably also muted simultaneously. anode saw-tooth voltage. This train of pulses Typical conditions which may arise and which at the Screen d is p yed for muting the will be assumed in the further description of the 9 rece ver. being fed to I. F. and L. F. stages thereof invention are as follows. over connection 8 and a valve stage (not shown) A representative radar equipment runs at 1000 for reversing their sense.

0. P. S. recurrence with a 1 microsecond pulse. The anode saw-tooth voltage is ap Over We require to raise the frequency of the intera coupling condenser C3 to the third grid of a ference to about 9000 cycles-in other words we gate Valve V2 mpr ng a pentode having use- 3 fully-short third-grid base. A diode V6 prevents the third grid of valve V2 being driven positive. Positive lock pulses from connection 6, and representing the interference pulses to be suppressed in the receiver, are applied to the first grid of valve V2.

The amplitude of the pulse appearing at the anode of gate valve V2 consequently depends on the relative timing of the lock pulse and the saw-tooth voltage from the transitron circuit. The anode pulse is rectified by a diode Vial-1d smoothed in a circuit RI, C4 to yield a positive bias voltage for the third grid of the transitron valve VI. The frequency of the saw-tooth voltage, initially adjusted to about 9000 C. P. S. depends upon the voltage to which the third grid of valve VI rises and this voltage is determined by the bias connection. Hence the frequency of the saw-tooth voltage is controlled according to the relative timing of the lock pulse and is increased or decreased as necessary to advance or retard the saw-tooth voltage with respect to the lock pulse and hence to maintain the timing of the muting pulses so that one of them symmetrically embraces the lock pulse and others are evenly distributed between succssive lock pulses.

More specifically, the voltage of the third-grid of valve V2 alternately rises slowly and falls rapidly giving rise to an inversely changing voltage at the anode of valve V2. If a lock pulse arrives late in the slow part of the cycle, the anode. voltage drop is great, the negative bias on the third grid of valve Vi is increased and the frequency of the transitron circuit is increased so that in a subsequent cycle the lock pulse occurs in the fast part. If it occurs early in that fast part, the anode voltage drop in valve V2 is again great and the transitron frequency is further increased. If it occurs late in that fast part, the anode voltage drop is smaller and the transitron frequency falls. Consequently the transitron circuit is controlled in frequency and incidentally in phase so that it is half way through a fast part of its cycle each time a lock pulse arrives. If the anode resistance or the third grid resistance in the transitron circuit is changed to alter the free running frequency, the gate valve and rectifier circuit pulls the frequency in steps from one multiple of the lock frequency to the next.

It will be noted that the lock pulse is not applied directly to lock the phase of the muting pulse generator. Such phase control is undesirable since it tends to synchronise the beginning or end of the muting pulse with the lock pulse whereas it is preferable that the muting pulse should embrace the lock pulse.

The invention may be applied to deal with any interference of a periodic nature provided that (a) the necessary multiplying factor is not excessive: as this factor is increased it becomes difficult after a number of additional muting pulses to ensure that the phase sequence is correct for the next interfering pulse; (b) there is not excessive jitter or irregularities in the interfering pulse; (a) the interfering pulse is not too wide to enable an interval to be provided for signal reception; if this interval becomes small, increased bandwidth in the receiver has to be provided.

I claim:

l. The method of reducing the interference caused in a communicationchannel by a series of intermittent sharp pulses of audible repetition rate comprising intermittently muting the received signals at a repetitionrate of a periodicity that will be substantially attenuated by the channel and which is higher than and a harmonic of said audible repetition rate, and synchronizing said muting with said pulses so that the received signals are muted for the duration of each sharp pulse.

2. The method of reducing radio interference caused by a series of sharp pulses comprising muting, the received signals for the duration of each. of said pulses, and further muting said received signals between each of said pulses; each of said muting acts being of short duration as compared to the spaces between adjacent muting acts.

3. The method of reducing interference in a radio receiver caused by a series of sharp substantially equally spaced pulses comprising muting the received signals for the duration of each of said pulses, and further muting the receiver a plurality of times between each of said pulses and at such a rate that all adjacent mutin acts are substantially equally spaced in time from each other.

4. The method of reducing interference in a muting pulses to cause a muting pulse to completely overlap each interference pulse, and biasing to cut-off both of said stages by said muting.

pulses.

5. In a communication channel subject to interference from an adjacent pulse generator which produces pulses at audible rate, means for receiving a fraction of the pulse energy from said generator, means connected to the output of the first-named means for generating a series of pulses at a'rate which is greater than and a harmonic of the interference pulse rate, and muting means to apply the output of the last-named means tomute said channel, the second-named means includingv means to synchronize a series of its pulses which constitute a submultiple of its repetition rate with pulses received at the firstnamed means.

a 6. In a radio receiver subject to interference from a nearby pulse generator, a radio frequency amplifier; a mixer connected to the output of said amplifier; an intermediate frequency am.- plifier connected to the output of said mixer; a second detector connected to the output of said last-named amplifier; an audio amplifier connected to the output of said second detector; means for generating a series of pulses at a rate which is a higher harmonic of the rate of the pulses of the pulse generator; means which receives energy from the pulse generator for synchronizing certain of the pulses of the firstnamed means with those of the pulse generator;

and means for blocking said radio frequency amplifier, said mixer, and said audio amplifier by said series of pulses.

7. In a radio telephone receiver, means for muting said receiver synchronously with interfering pulse signals having components of frequency below the upper audible limiting frequency of said receiver, and means for muting the.

receiver during further short intervals of time so that any two successive muting operations are '5 separated by a time which is less than the reciprocal of said limiting frequency.

8. In a communication system including a telephone channel receiver liable to interference by interfering pulse signals having frequency components below the upper audible limiting frequency of the receiver, means for muting said receiver during each occurrence of an interfering pulse, and means for muting said reeciver during further short intervals so that the whole muting operation introduces no frequency below said limiting frequency.

9. The method of reducing interference in a superheterodyne receiver comprising the steps defined in claim 3 in which the muting step is accomplished prior to the step of amplifying the intermediate frequency.

10. The device defined in claim 5 in which said channel comprises a radio frequency amplifier stage, a mixer stage, at least one intermediate frequency amplifier stage, and an audio amp1ification stage, and in which said muting means comprises biasing means connected to negatively bias at least one of said stages that is ahead of the first intermediate frequency amplifier stage.

11. The method defined in claim 1 in which the muting acts have greater time duration than the received pulses; and in which the synchronizing step causes muting to begin prior to the start of each sharp pulse, to continue for the duration of each such sharp pulse, and to continue for a limited time after the cessation of each such sharp pulse.

ALAN JOHN HENRY OXFORD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,096,874 Beers Oct. 26, 1937 2,151,740 Burrill Mar. 28, 1939 2,209 507 Campbell. July 30, 1940 2,250,144 Welty July 22, 1941 2,265,290 Knick Dec. 9, 1941 2,408,192 Bell et a1 Sept. 24, 1946 2,420,303 DeFrance May 13, 1947 2,425,315 Atwood Aug. 12, 1947 2,459,798 Dettman Jan. 25, 1949 2,460,202 Tyson Jan. 25, 1949 

